Convert most of remaining ports that depend on xorg-libraries to
modular xorg.
- supply corresponding USE_XORG for all imake-using ports that need it
- USE_IMAKE no longer implies USE_XLIB in absence of USE_XORG
- retire USE_X_PREFIX which is not really used anywhere after the
above change
- a few minor nits like whitespace and SF macro
Tested by: 2 tinderbox runs by pav
Approved by: portmgr (pav)

- Add the X_WINDOW_SYSTEM={xorg,xfree86-4,xfree86-3} variable to bsd.port.mk,
and make XFREE86_VERSION map to it. XFREE86_VERSION is now deprecated.
- Make xorg the default X_WINDOW_SYSTEM on -current.
- Add several new X_*_PORT variables which point to various pieces of X11 based
on the setting of X_WINDOW_SYSTEM, and make ports use them.
- Add information to CHANGES about how to handle the transition.
PR: ports/68763
Approved by: portmgr (marcus)
Approved by: re (scottl)

Recently I updated from Wnn 4.2 to FreeWnn 1.1.1. Then include files
${LOCALBASE}/include/wnn/*.h and a archive librarry libwnn.a of FreeWnn
conflict with ones of Wnn6-lib. So I move Wnn6-lib's include files into
${LOCALBASE}/include/wnn6/wnn and remove libwnn.a of Wnn6-lib. Note that
libwnn6.a can be used instead of libwnn.a for Wnn6-lib.